The present invention relates to a magnetic bearing device concurrently utilizing a rolling bearing unit and a magnetic bearing unit and utilizable in, for example, a turbine unit in an air cycle refrigerating system, in which the magnetic bearing device is operable to support one or both of an axial load and a bearing preload.
The air cycle refrigerating system makes use of an air as a refrigerating medium and does therefore fail to exhibit a sufficient energy efficiency as compared with the refrigerating system using chlorofluorocarbon or ammonium, but is considered agreeable in terms of environmental protection. Also, in a facility such as, for example, a cold storage warehouse into which a refrigerating air can be directly blown, the total cost can be lowered if a cooling fan device and/or defroster are dispensed with, and, accordingly, the use of the air cycle refrigerating system in such application has been suggested. (See, for example, the Japanese Patent No. 2623202.)
Also, it is well known that in a deeply cold range of −30 to −60° C., the theoretical efficiency of air refrigeration is equal to or higher than that of chlorofluorocarbon or ammonium. It is, however, said that optimally designed peripheral equipments are needed in order to secure the theoretical efficiency of the air refrigeration. Those peripheral equipments include, for example, compressors and/or expansion turbines.
For the compressor and expansion turbine, a turbine unit, in which a compressor rotor and an expansion turbine rotor are mounted on a common main shaft, is generally utilized. (See, for example, the Japanese Patent No. 2623202.)
It is to be noted that for the turbine unit used to handle a process gas, a magnetic bearing type turbine unit has been suggested, in which the turbine rotor and the compressor rotor are respectively mounted on respective opposite ends of the main shaft, which is supported by a thrust bearing and a journal that can be controlled by an electric current flowing through an electromagnet. (See, for example, the Japanese Laid-open Patent Publication No. 07-91760.)
Also, although related to a suggestion concerning a gas turbine engine, the use of a thrust magnetic bearing device has been made to reduce the thrust load, acting on the rolling bearing device for the support of a main shaft, in order to avoid the possibility that the thrust load would lead to reduction in bearing lifetime. (See, for example, the Japanese Laid-open Patent Publication No. 08-261237.)
As discussed above, in order to secure the theoretical efficiency of the air cooling, at which a high efficiency can be obtained in the deeply cold range, the air cycle refrigerating system requires the use of a compressor and/or an expansion turbine that are optimally designed.
For the compressor and the expansion turbine, as mentioned above the turbine unit including the compressor rotor and the expansion turbine rotor both mounted on a common main shaft is utilized. This turbine unit increases the efficiency of the air cycle refrigerator in view of the fact that the compressor rotor is driven by a power induced by the expansion turbine.
However, in order to secure a practically acceptable efficiency, a clearance delimited between each of the rotors and a housing must necessarily be small. Change in clearance constitutes a cause of an unstable operation during high speed rotation and, therefore, the efficiency tends to be lowered.
Also, by the effect of an air acting on the compressor rotor and the turbine rotor, the thrust force acts on the main shaft and the bearing unit supporting the main shaft is loaded with the thrust load. The rotational speed of the main shaft in the turbine unit employed in the air cycle refrigerating system is 80,000 to 100,000 revolutions per minute, which is considerably high as compared with that in the bearing unit for the standard application. For this reason, the thrust load such as described above tends to bring about a reduction in long-term durability and lifetime of the bearing unit used to support the main shaft and, in turn, a reduction in reliability of the air cycle refrigerating turbine unit. Unless the problem associated with the long-term durability of the bearing unit is resolved, the air cycle refrigerating turbine unit can be hardly placed in practical use. However, the technology disclosed in the Japanese Patent No. 2623202 has not yet resolved the problem associated with the reduction in long-term durability of the bearing unit relative to the loading of the thrust load under such a high speed revolution.
In the case of the turbine compressor of a magnetic bearing type such as disclosed in the Japanese Laid-open Patent Publication No. 07-91760, in which the main shaft is supported by the journal bearing unit in the form of a magnetic bearing and the thrust bearing unit, the journal bearing unit lacks a function of supporting in the axial direction. For this reason, the presence of a factor or the like that render the control of the thrust bearing unit to be unstable makes it difficult to achieve a stabilized high speed revolution while the minute clearance is maintained between the rotor and the diffuser. The magnetic bearing unit involves a problem associated with a contact between the rotor and the diffuser at the time of failure of the electric power supply.
Where arrangement is made that the rolling bearing unit and the magnetic bearing unit are concurrently used to support the main shaft, and the rolling bearing unit is operable to support the radial load whereas the magnetic bearing unit is operable to support one or both of the axial load and the bearing preload, the main shaft can be supported in the axial direction and there is no problem associated with the contact at the time of failure of the electric power supply such as occurring in the magnetic bearing unit. In the case of this construction, the electromagnet of the magnetic bearing unit is provided so as to confront, without contact, the flange shaped thrust plate coaxially-mounted on the main shaft so as to extend perpendicular thereto and made of the ferromagnetic material, and the electromagnet is controlled by the controller in response to the output from the sensor for detecting the force in the axial direction.
When the main shaft of the rotor is supported by the rolling bearing unit, the position of the main shaft can be regulated in the axial direction by the regulating function of the rolling bearing unit and the clearance between the rotors and the housing can be maintained at a constant value. In the case of the rolling bearing unit having the function of regulating the position in the axial direction as discussed above, reduction in long term durability due to the thrust force in the bearing unit then revolving at high speed poses a problem, but since the thrust force is supported by the electromagnet, the long term durability of the rolling bearing can be secured.
However, in the magnetic bearing device, in which the rolling bearing unit and the magnetic bearing unit are used concurrently, in the event that an excessive axial load acts, the negative stiffness of the electromagnet (which acts in a direction of displacement and as the displacement increases, the force thereof increases correspondingly) comes to be large. When the negative stiffness of the electromagnet comes to be higher than the stiffness of the composite spring formed by the rolling bearing unit and the support system of the rolling bearing unit, the control system of the magnetic bearing device becomes unstable. In order to avoid such condition, there is a need to add a phase compensating circuit to the controller beforehand, resulting in a problem that the controller tends to be complicated in structure.